Find ${ \mathbf a \cdot \mathbf b }$ if $\left| {\mathbf a} \right|$ = 3, $\left| {\mathbf b} \right|$ = 6, and the angle between ${\mathbf a}$ and ${\mathbf b}$ is $- \frac{\pi}{7}$ radians.

${ \mathbf a \cdot \mathbf b }$ =